Unfortunately, they're really popular, and seem to be receiving a lot of mainstream press as of late. A quick google search turns up the following:
What a cursory search for "peel-off mask" returns
Why are these masks so popular? In a word, it's novelty. Prior to peel-off masks becoming popular, most masks have been clay or gel masks that you leave on and wash off, and can often get messy and drippy. By comparison, peel-off masks seem so much more fun and easier to use - the mask doesn't drip, and instead of washing off the mask, you get the satisfaction of peeling off the mask like this:
And, now brands are adding to the fun and entertainment factor of peeling masks by doing things like adding glitter to them, which while fun, doesn't really enhance the skincare value of the product in anyway. But, damn if it doesn't look like the most fun to apply and peel off though!
But as I've mentioned (right in the title, too), I'm not a huge fan of peel-off masks. I personally find them really bad for skin, and the general formula of these peel-off masks is such that by their nature, they're rather drying and sensitizing to the skin, and it's hard to escape that fact, no matter how much glitter you throw into them. We'll take a look at the general formula of such masks, and see how the masks work, and then check out the ingredients lists of some of the more popular peel-off masks, to see how much they adhere to the general formula. It's lots of ingredients reading. But it's fun! And worth it for better skin!
